pps. the *error* is of course that
the author of the supposed solution
has evidently failed to *check the work*
after computing the value of y.
(the check is *part of the solution*
since even correct computation
can lead to so-called “extraneous
solutions”… with any luck at
least *some* students will
mention this… this is a good
exercise!).

oh, and… what the heck,
[[3,2][-4,1]]'[[4][9]]Frac—>
oughta get part two
(where ‘ denotes ^{-1})
not that i’ve checked it.
tools at hand!